Butterfly XML 1.1: A Complete, Professional Strength Open Source XML Editor
http://www.butterflyxml.org/

Butterfly XML Editor is an IDE built on top of a new real-time incremental XML parsing algorithm. The editor features syntax and error highlighting, incremental validation, code completion, XSLT pipelines, and side by side DOM and source viewing. Their Mission is to provide a cutting edge XML IDE for developers who demand professional strength software. This has been added to World Wide Web Reference Subject Tracer™ Information Blog.